A conventional loudspeaker produces sound by using a moving diaphragm or cone to create pressure waves in the surrounding air. As the diaphragm moves forward, it creates a high pressure peak at its front, and a low pressure trough at its back, such that the front wave and back wave are 180 degrees out of phase. If these waves are allowed to merge and mix together, there will be randomly distributed constructive and destructive wave interference at various wavelengths and locations, resulting in an uneven sound field in the listening area. Loudspeakers are typically mounted in some type of baffle or enclosure designed to keep the front wave and back wave separate. This becomes more important at the lower end of the frequency range where the wavelengths are several feet in length. It also becomes more difficult because at the lower frequencies, the enclosure needs to be larger to accommodate the longer wavelengths.
At some frequency, a speaker enclosure will become a resonant chamber, and will get much louder, while the amplitude of the driving signal is held constant. A transient may even cause it to ring at it's resonant frequency. Thus, it will greatly color or distort the sound. Packing the enclosure with fiberglass batting, or lining the interior walls with acoustic foam can reduce these effects, but enclosures typically cannot function effectively at or below resonance.
The two most common low frequency speaker enclosures are the “bass reflex” type (also known as “tuned port”), and the “acoustic suspension” type. A bass reflex box can extend its low frequency range by phase shifting the back wave through a delay path so it partially cancels the front wave at the resonant frequency, but constructively adds to the front wave at some frequency below the natural resonance. However, the resulting sound is often described as “boomy” near the center frequency of the tuned port, or muddy at other frequencies, where the phase shifting is not optimal. Also, the suspension of the cone has to be somewhat stiff to control the excursion during transients and loud bass notes. The characteristics of the physical suspension can color the sound and produce unwanted artifacts.
An acoustic suspension unit is inherently less efficient because only the front wave leaves the box. The back wave is absorbed and dissipated inside the box. However, an acoustic suspension enclosure typically produces cleaner sound with less distortion. Since the enclosed air acts as a perfect spring to control the cone excursion, the speaker cone can use a softer more compliant suspension. In spite of the better sound quality, acoustic suspension speakers have recently been less popular, because until now, the only way to lower the resonant frequency of such an enclosure was to make it physically larger.